The present invention relates to an optical time-domain reflectometer (OTDR) for detecting and monitoring losses and faults in optical fibers employed, for example, in a dense wavelength division multiplexing (DWDM) communication system.
Long distance communication systems are upgrading to the use of dense wavelength division multiplexing (DWDM), allowing optical channels to be spaced a few nanometers or less. Although greatly increasing transmission capacity, as these changes are made, significantly more capacity and investment are placed on a single optical fiber, warranting continuous monitoring for losses or faults in the fiber, such as through remote testing.
It is well known in the art, however, that an optical time-domain reflectometer (OTDR) can be used to locate faults, or to measure transmission loss in an optical fiber. More specifically, an OTDR launches a test pulse of light into the optical fiber, and then monitors the back-scattered light for changes in intensity, indicative of a loss or fault.
To ensure reliable fault coverage, active fiber OTDR testing is currently being deployed on DWDM communication systems to monitor for losses and faults. Unfortunately, it has not been recognized that the current OTDR method is generally ill-suited for DWDM communication systems.
In accordance with the principles of invention, an optical time-domain reflectometer (OTDR) employs a so-called xe2x80x9cout-of-bandxe2x80x9d offsetting to cancel the effects of Raman non-linearities which extract energy from the traffic signal wavelengths and amplify the test signal back-scattering, thereby corrupting the ODTR measurement. Losses and faults in the optical fibers are monitored by measuring the back-scattered portion of the light launched into the fiber, but the test signal back-scattering is judiciously offset to account for Raman non-linearities. That is, the effects of the Raman non-linearities are taken as a baseline measurement and, then accordingly used as the basis to offset the back-scattered signal.
This latter xe2x80x9cout-of-bandxe2x80x9d offsetting is accomplished by first measuring the xe2x80x9cout-of-bandxe2x80x9d back-scattering without the presence of a test signal, but while there is still live traffic. More specifically, just prior to launching a test light pulse, the OTDR first measures the xe2x80x9cout-of-bandxe2x80x9d back-scattering due to the Raman non-linearities, that is the amount of back-scattering reaching the ODTR that occurs at the test signal wavelength due to stimulated Raman scattering. This xe2x80x9cout-of-bandxe2x80x9d back-scattering is measured by generating a xe2x80x9cblankxe2x80x9d test pulse, and then measuring the backward-scattering occurring at the test wavelength.
Once the xe2x80x9cout-of-bandxe2x80x9d or baseline back-scattering has been obtained, the OTDR launches a short duration test light pulse(s) into the optical fiber, and measures as a function of time the test signal back-scattering, which is the sum of the back-scattering due to Rayleigh scattering and possibly Fresnel reflections, but amplified due to the Raman gain, G. Importantly, the previously measured xe2x80x9cout-of-bandxe2x80x9d or baseline back-scattering occurring at the test signal wavelength is used to calculate the Raman gain, G, which in turn is used as the basis to offset the test signal back-scattering, thereby canceling the effects of the Raman non-linearities.